In lasers such as those disclosed in U.S. Pat. Nos. 5,457,709 and 6,751,244, electrons are extracted by resonance with one or two optical phonons. In U.S. Pat. No. 5,457,709, the electrons can occupy several subbands in each well, e.g., an upper subband, a middle subband, and a lower subband. The upper subband F corresponds to the injection energy of the electrons. The active electrons start from this subband. These electrons generate laser radiation upon passing into the middle subband G. In order for stimulated emission to take place, it is necessary for the population of the upper subband F to be greater than that of the population of the middle subband G, which can be obtained by resonance with an optical phonon. This resonance may be obtained provided that the energy difference between the middle subband G and the lower subband H corresponds, upon transition of an electron, to an energy loss EGH which is substantially equal to the energy of the optical phonons emitted by the single crystal material of a well.
U.S. Pat. No. 6,751,244 discloses that the properties of a laser may be improved by using resonance with two phonons. In the disclosed configuration, the electrons may occupy a fourth subband J, called the second lower subband. The energy difference between the first and second lower subbands H and J corresponds, upon transition of an electron, to an energy loss EHJ which is substantially equal to EGH and, consequently, to the energy of the optical phonons emitted.